You have to go to the drop-down menu on top of the steam client labeled "Games", then click on "Activate a Product on Steam"...this will bring up the prompt to type out the key you've been given.

All I know is this Lo-Pan character comes out of nowhere...while his buddies are flying around on wires cuttin' everybody to shreds and he just stands there, waiting for me to drive my truck straight through him with light coming out of his mouth?

Start up steam. Navigate to games.(Hover over library) On the bottom there should be a button that says +add a game.Click this and select (activate a product on steam). From there just press 'ok' or 'I AGREE'. After all that It will ask for youre key that you got in youre email. Hope it works and I didnt miss a step if it didnt. then just ask steam adminastrators.